This invention relates to a digital time division, multiplex telecommunications switching system, and more particularly to an ADA test for providing real time, on line, fault detection and diagnosis of faults in analog-to-digital (A/D) sections and digital-to-analog (D/A) sections of code converters in the switching system.
In general, call processing by a telecommunications switching system is accomplished by connecting a terminal, for transmitting and receiving information, to another terminal by means of a switching network. In a digital time division multiplexed telecommunications switching system the switching network, which includes an information memory, connects one terminal to another terminal "in time" by sequentially storing digital samples of information from first and second terminals in assigned locations of the information memory, swapping the two samples in time and returning the first and second samples respectively to the second and first terminals.
In providing "in time" call processing between analog information terminals, such as telephone instruments, the information transmitted by the terminals must first be sampled (to produce pulse amplitude modulated, PAM, signals) and digitally encoded by analog to digital (A/D) sections of code converters. The resulting successive digital samples (multi-bit binary words which represent data viewable as numbers) from each terminal correspond to a particular channel in the switching system. After sampling and encoding, the digital samples, at A/D section digital outputs, are time multiplexed by multiplexers in order to put several channels of information onto a single transmission path or wire with each channel occupying a dedicated time slot. Whenever a channel is idle, its time slot on the wire will simply be vacant. Time multiplexing of a number of channels serves to minimize the number of physical wires going to the information memories in the switching network. After the swapping has occurred in memory, the information is routed in reverse sequence back through demultiplexers, to digital-to-analog (D/A) sections of code converters and via D/A section analog outputs to the connected terminal.
The swapping of samples in the information memories is accomplished under computer control. The computer or control complex (abbreviated as "CPU") also keeps a record of the status of the switching system including the state of the terminals so as to properly process a request for service. In a system using the present invention, the computer may be distributed microprocessors (as described in Pitroda et al. U.S. application Ser. No. 842,091 filed Oct. 17, 1977 which is a continuation-in-part of Ser. No. 734,732, filed Oct. 21, 1976 and now abandoned) which microprocessors control call processing and monitor the status of the system.
In a digital time division, multiplex switching system of a practical size, having as many as three thousand channels, the number and the complexity of information routing paths or loops to the switching network and "time" paths within the switching network produce a complicated maintenance and diagnostic problem. If, for example, there is a failure in a switching element such as in the connection memory, the wrong channel samples may be swapped. As a second example, malfunction of a voice bit in an A/D or D/A section of a code converter may result in distorted sound transmission of one terminal to another. Any such failure appears to be a fault in the information loop to and from the connection memory including the code converters and the multiplexers/demultiplexers. The maintenance and diagnostic problem becomes how to determine whether there is a real hardware failure in the code converters or the multiplexers/demultiplexers or only an apparent failure in the code converters or the multiplexers/demultiplexers which apparent failure results from "in time" connection failure in the connection memory.
Furthermore, telephone switching equipment must have an extremely high degree of reliability. Commonly, only one hour of system down time in twenty years is the established reliability goal. It is important, therefore, to detect and identify faults as soon as they occur, so that they may be cured expeditiously or back-up equipment placed in service.
Maintenance of the switching system hinges on providing fault detection which is able to rapidly detect real, as opposed to apparent, faulty operation of the system components so that proper remedial action can be taken, i.e. switching in spare circuits or providing fault information so that a craftsperson can replace the faulty components.